## Identification of S. pneumoniae Among Alpha-Hemolytic Streptococci **Key Point:** *Streptococcus pneumoniae* is uniquely identified among alpha-hemolytic streptococci by its sensitivity to optochin and solubility in bile — these two tests are the gold standard for presumptive identification. ### Optochin Sensitivity Test **High-Yield:** Optochin (ethylhydrocupreine) inhibits the autolytic enzyme of *S. pneumoniae*, causing lysis of the organism. A zone of inhibition around an optochin disk indicates *S. pneumoniae*. - **S. pneumoniae:** Optochin-sensitive (inhibition zone ≥14 mm) - **S. viridans and other alpha-hemolytic streptococci:** Optochin-resistant (no inhibition) ### Bile Solubility Test *S. pneumoniae* possesses an autolysin that is activated by bile salts, causing the organism to lyse and disappear from the culture. - **S. pneumoniae:** Bile-soluble (colonies dissolve in bile) - **S. viridans:** Bile-insoluble (colonies persist) ### Differential Identification Table | Test | S. pneumoniae | S. viridans | S. bovis | |------|---------------|-------------|----------| | Hemolysis | Alpha | Alpha | Alpha | | Optochin sensitivity | **Sensitive** | Resistant | Resistant | | Bile solubility | **Soluble** | Insoluble | Insoluble | | Lancefield antigen | None | None | Group D | | Catalase | Negative | Negative | Negative | **Mnemonic:** **PONS** = **P**neumo **O**ptochin **N**egative **S**viridans (i.e., viridans is optochin-negative; pneumo is optochin-positive) **Clinical Pearl:** In clinical laboratories, the combination of alpha-hemolysis + optochin sensitivity + bile solubility is sufficient for presumptive identification of *S. pneumoniae* without further testing. Confirmation can be done by MALDI-TOF or 16S rRNA sequencing. [cite:Koneman's Color Atlas and Textbook of Diagnostic Microbiology 7e Ch 14]
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